1. Field of the Invention
The present invention generally relates to semiconductor devices, and particularly relates to semiconductor devices having a high-K dielectric film of metal oxide or metal silicate, and a fabrication process thereof.
2. Description of the Related Art
Since semiconductor integrated circuit devices, such as CMOS LSI, are required to operate at an ultra high-speed, the devices are structured by electric field effect type transistors (MOSFET) that have a very short gate length. To attain the ultra high-speed operations, intensive efforts have been made on miniaturization of MOSFET.
In miniaturization of MOSFET, thinning of a gate insulation film is also required. For example, decreasing film thickness of the gate insulation film to about 2.5 nm or less in terms of oxide equivalent thickness is called for.
Conventionally, silicon oxides having excellent leakage characteristics and small interface level density have been used for the gate insulation film of MOSFETs. However, the conventional gate insulation film of silicon oxide suffers from the problem of increased direct tunnel current, as the physical thickness of the gate insulation film gets thinner. Any film thickness less than the value described above will cause a paramount problem of the gate leakage current due to the direct tunnel current. If the gate leakage current is increased, a substantial leakage current keeps flowing during a gate-off period, causing problems, such as abnormal operations and higher power consumption.
In order to solve the problems, the use of high-K dielectric films, such as metal oxide metals and metal silicates, has been studied. In the present invention, a high-K dielectric film is used in the sense to indicate a dielectric film having a specific dielectric constant of 10 or more.
However, in the high-K dielectric gate insulation film that employs metal oxide or metal silicate, there arises a problem of unstable operations of MOSFET, due to a phenomenon that boron doped to the gate electrode penetrates to the high-K dielectric gate insulation film, and hydrogen in gas used in manufacturing processes attacks the high-K dielectric gate insulation film. Further, there is a possibility that reactions, such as silicide formation, may arise at an interface between a silicon substrate and the high-K dielectric gate insulation film or at an interface between the high-K dielectric gate insulation film and a poly silicon gate electrode, when forming a gate structure containing the high-K dielectric gate insulation film on the silicon substrate surface.
Conventionally, studies on the high-K dielectric gate insulation film have primarily been conducted in relation to suppressing the direct tunnel current, and studies about stability of the device characteristics have not been extensively conducted.
For example, JP, 2001-267566 discloses a gate insulation layer that consists of a first SiN molecular layer formed by a so-called single atomic layer depositing (atomic layer CVD) method on a Si substrate surface, a high-K dielectric layer such as a ZrO2 layer formed on the SiN molecular layer by repeatedly forming an oxygen atomic layer and a Zr atomic layer, each formed by the atomic layer CVD method, and a second SiN molecular layer formed on the high-K dielectric layer by the atomic layer CVD method. Further, the related technology also discloses a gate insulation layer that consists of a first SiO2 molecular layer formed by a so-called single atomic layer depositing (atomic layer CVD) method on a Si substrate surface, a high-K dielectric layer such as a ZrO2 layer formed on the SiN molecular layer by repeatedly forming an oxygen atomic layer and a Zr atomic layer, each formed by the atomic layer CVD method, and a second SiO2 molecular layer formed on the high-K dielectric layer by the atomic layer CVD method.
The dielectric gate insulation film such as above has small oxide equivalent thickness, and can effectively suppress the gate leakage current caused by the direct tunneling effect when applied to an ultra high-speed semiconductor device having gate length of 0.1 micrometers or less.
However, with the structure in which the first and the second SiN molecular layers vertically sandwich the high-K dielectric film, the Si substrate surface cannot be uniformly and completely covered by nitrogen atoms due to a difference between the valence of Si and nitrogen, but a dangling bond will inevitably arise. If a dangling bond arises in the gate insulation film, especially in the interface with the Si substrate surface, which serves a channel region, threshold characteristics of the semiconductor devices are changed by a carrier trap and the like.
In the case of the foregoing prior art in which the high-K dielectric film is vertically sandwiched by a pair of SiO2 molecular layers, on the other hand, there are no dangling bond formed at the interface between the Si substrate and the gate insulation film. However, because of absence of the nitrogen atomic layer in the gate insulation film, there arises a problem that the B dopant in the polysilicon gate electrode spreads into the Si substrate through the gate insulation film, and threshold characteristics of the semiconductor device are changed. Further, in such a high-K dielectric gate insulation film that lacks the nitrogen atomic layer therein, there arises a problem that oxygen in the high-K dielectric film easily reaches the Si substrate by diffusion and causes degradation of carrier mobility in the channel region. Further, there may be caused a problem of diffusion of metal element such as Zr to the Si substrate from the gate insulation film. The metal element thus reached the Si substrate may cause a reaction such as silicide formation.
Thus, the conventional high-K dielectric gate insulation film has suffered from the problem of dangling bond formation at the interface between the Si substrate and the high-K dielectric gate insulation film, and the problem of penetration of impurity element through the high-K dielectric gate insulation film. Further, the problems of diffusion of oxygen and metal elements are also unresolved. Thus, the expected advantageous effect of the high-K dielectric insulation film has not been achieved even when the semiconductor device is produced by using a high-K dielectric gate insulation film.